Third Generation (3G) Networks such as UMTS (Universal Telecommunication Network) and CDMA 2000 provide high-speed wireless Internet access to mobile users over a wide coverage area. For the 3G networks the IP Multimedia Subsystem IMS, standardised by the Third Generation Partnership Project (3GPP), has been defined to support voice and multimedia services. The strength of IMS is the provision of enhanced services, for example multimedia services combining voice and data over packet-switched technology, in particular IP-network. The usage of IP-network as a single underlying standard allows an easy and fast service deployment. Furthermore the IMS is also a network architecture, servicing both fixed and mobile terminals which seamlessly integrates into the ubiquitous IP environment, including the Internet and other packet-switched based networks.
A number of mobile communication networks, such as GSM, are based on a circuit-switched technology, wherein both the access domain and the service domain are based on circuit-switched technology.
Due to the fact that service providers shift their core network infrastructure from the circuit switched domain to a consolidated common packet switched infrastructure, the need occurs to enable the consistent provision of services to subscribers over a variety of accesses domains including circuit-switched CS and packet switched PS accesses and different service domains. Therefore common controlling platforms are introduced, like the IP Multimedia Subsystem (IMS) for providing enhanced services for the multitude of different service domains that may be available for an individual call and between which the user might be switched, wherein it is to be ensured that choice of a service domain remains transparent to the user.
A solution for consistently routing calls from a CS network access through the IMS network to the IMS service domain is currently investigated by the 3GPP. The work item is called IMS Centralized Services (ICS) and aims at moving all subscribers to the IMS for a harmonization of the service (see 3GPP TSG SA WG2 Architecture—SA2#5, Sophia Antipolis, France, 28 Aug. to 1 Sep. 2006, Tdoc S2-063335).
The aim of the ICS is to be able to handle users accessing or being served by different service domains, like for example to handle a user having a subscription in a first domain, like IMS, and being in a coverage of a second domain, like for example circuit-switched domain.
The ICS concept requires the installation of the so called ICS client application providing the functionality for routing between domains. Currently, two alternatives for the placing of the ICS client application are investigated. According to the first approach, network-centric concept, the ICS client application is introduced in the network, for example on a Mobile Switching Centre (MSC) or on an MSC server (MSC-S). The second approach proposes to introduce the ICS client application in the terminal.
In 3GPP TSG SA WG2 Architecture, 28-30 Mar., 2006 in Munich, “USSD based call flows” a mechanism for attaching an IMS user via a legacy network is mentioned. It describes to use a Voice Call Continuity VCC application, for combining CS and PS functionality. The VCC application is an IMS functionality located in an IMS node in the IMS domain. Therefore the cited document proposes to integrate the functionality for the translation between the CS domain and the IMS domain into an internal node of the IMS domain. Consequently the IMS has to distinguish whether a user is an IMS or a CS user in order to handle them in a different manner. Further therein it is described to use Unstructured Supplementary Service Data (USSD) in the CS domain for attaching an end terminal to the VCC application. However the cited document does not provide a complete solution for performing IMS registration after a CS attached procedure is finished.